A Rate Integrating Fiber Optic Gyro

Abstract

In the past, fiber optic gyros have generally been viewed as angular rate sensors. However, for inertial navigation purposes, a “rate integrating” gyro characteristic is desired. The distinction is subtle. but important. A rate gym’s output represents an estimate of instantaneous angular rate. The system attitude is determined by sampling the rate and integrating numerically to angle. On the other hand, a rate integrating gyro provides the change in rotation angle rather than instantaneous rate. The system accumulates the angular incremenu obtained from the gyro to determine rotation angle and, hence, attitude. A ring laser gym is a good example of a rate integrating sensor. Its output consists of pulses which represent fixed angle increments. The rate integrating characteristic is particularly important for navigation in vibration environments. A system subjected to wideband random vibration inputs will only exhibit bounded angular excursions. Yet instantaneous angular rates will remain random over a substantial frequency range. If a rate gyro is periodically sampled and the instantaneous rate integrated numerically to provide angle (and, hence, attitude). a random walk process will occur. That is, as the random rates are added together. the angle error grows unbounded in tbe same way as it does in response to actual gyro angular random walk. This angle drift can be very large in severe vibration environments. For this reason, a fiber optic gyro configured as a rate sensor may appear ill-suited for navigation purposes. Howevex, a rate integrating gyro does not suffer from the pmblem of vibration-induced angular random walk. Because the actual angle always remains bounded and since the rate integrating gyro truly measures angle, the. random drift incurred by a rate gyro is avoided. Thus. we have developed a rate integrating tiber optic gyro mechwiion. The paper discusses the thwry and sppucation of this concept.